Dental implantology is an evolving science featuring many different implant designs. The main objective of each implant system is to provide strong and immediate fixation in the bone for the support of bridge work and other tooth prostheses. In order to establish this support, it is necessary for the dental implants to osseointegrate with the bone tissue. This osseointegration, however, has been illusive and difficult to achieve since bone does not bridge in space or move toward the implant readily, especially since connective tissue moves 0.5 mm a day following surgery and bone may take up to three months, hindered by connective tissue in the process. Hence, many different implant designs and implant techniques have been devised for the osseointegration concept.
One of the more successful dental implants systems using the process of osteocompression features a screw-type implant having a rounded thread. This implant is illustrated in U.S. Pat. No. 5,007,835, issued to Maurice Valen on Apr. 16, 1991, for DENTAL IMPLANT, incorporated herein by way of reference. The screw-type implant is threaded into a drilled and tapped hole that is prepared in the jaw bone. The osseous drilled hole is undercut by automated surgical tapping instrumentation having compound angles for bone compaction, rather than bone removal as with prior art. The rounded threads of the implant compress against the undercut surface of the tapped osteotomy, thus causing a radial osteocompressive force and bone lamination.
It has been found that this aforementioned screw-type implant has a major advantage over other implant designs due to the method of osteocompression by the implant design at the bone site. This advantage resides in the ability of the implant to provide immediate fixation in the bone within physiologic limits, placing the implant into immediate clinical function. The implant design dramatically increases the distance between the major diameter and the minor diameter of the implant, thus providing greater load bearing areas of implant support in horizontal planes at osseous sites. The radial force gives an immediate horizontal supportive function and laminates bone similar to the lamina dura of a natural tooth. This makes possible the instantaneous attachment of a tooth or a bridge superstructure to the implant abutment after surgery, placing the implant into immediate function by the patient and avoiding the loss of the laminated bone due to disuse atrophy.
This immediate functionality by design and novel surgical instrumentation clearly contrasts with blade, conventional screw, or push-in implants that often require a year or more for the bone to osseointegrate with the implant before becoming clinically functional to the patient.
More recently, it has been determined by animal studies that the radial compressive forces provided by an osteocompressive implant against the wall of the threaded bone tissue stimulate the layer of bone surrounding the prepared osteotomy. This stimulation nourishes the osteoblast cells of the bone, causing them to lay down new bone formation as demonstrated by histologic models in many renown scientific laboratories. Given occlusal force magnitude, the compressive forces exerted at the implant thread region have minimized the tension throughout the implant interface by the increase of implant region in horizontal planes and the improved bone quality at thread region by lamination. Only the radial forces provided by controlled osteocompression have been found to produce this bone stimulation and controlled functional osteocompression. Animal studies did not demonstrate bone necrosis in any of the implant sites. Conversely, animal studies for a saw-tooth-like implant did demonstrate minimal success due to sharp edges of implant areas or the lack of intimate bone contact at implant interface due to the concept of osseointegration.
The present invention seeks to improve the already successful screw-type dental implant mentioned above. It is known that bone strength varies as the bone density is different in parts of the maxillary and mandibular bone regions. The varying bone strengths make necessary the changing of the screw thread pitch to accommodate particular jaw sites based on bone quality and forces generated by the patient. This invention reflects the discovery that the pitch of the screw thread of the implant must be increased in weaker bone site areas. Whereas a lesser undercut in the bone caused by the primary tap is provided for the maxillary bone as compared to mandibular bone, attaining more support and compression for the weaker bone by the knowledge of known mechanical properties of bone and the increase by the bulk modulus of the bone against known implant load bearing areas for radial osteocompressive action by the threads the day of placement. Relatively speaking, the mechanical relationship between implant and bone can be defined by the comparative moduli of elasticity. The modulus of elasticity of trabecular bone is approximately 1.5.times.10.sup.6 psi (one and one-half million), while the modulus of elasticity of cortical bone is approximately twice that of trabecular bone. The screw-type implant of the present invention is fabricated from titanium, and has a modulus of elasticity nearly five times as great as cortical bone, and ten times greater than trabecular bone where all of the endosseous implant resides. The forces imparted to the bone by the titanium implant must be in equilibrium for the implant to be successful. The size of the implant threads are also chosen relative to, and therefore dependent on, increasing the bulk modulus of bone to improve the bone's mechanical properties by controlled radial osteocompression and bone lamination.
In order to achieve dynamic balance between the implant and the weaker bone structure, it becomes necessary to provide sufficient force bearing areas of implant support in horizontal planes since implants are not physiologic, that is to say there is no physiologic attachment mechanism nor bone lamination to implant interface as with natural teeth for bone stimulation. Due to tooth extraction and disuse osseous atrophy, the weaker bone areas require more bulk support of bone in order to accommodate the disparity in the elastic moduli between the bone site and the implant interface under mastication. Ironically, the weaker bone areas of the jaw, such as in the maxillary molar regions, are also those areas sustaining greater masticatory forces.
In accordance with the present invention, greater bone support is achieved by providing a greater distance between the major and minor diameters and by increasing the pitch of the screw inversely to the bone modulus of elasticity for bone lamination and osteocompression within physiologic limits. The increased pitch allows more bone volume between implant threads, thus providing increased support in weaker bone areas. The opposite is also true, when bone in stronger (i.e., symphyseal bone) the pitch of the screw thread can be reduced as noted with the present invention for controlled radial force lamination by rounded screw threads without compromise to vascularity by over-compression.